Gantry systems can be widely used for a range of applications, from assembly and electronic manufacturing, to vision systems and industrial automation. In semiconductor assembly and packaging, positioning systems using gantries are useful where a work space spans a predefined area and it is necessary to position a device accurately at various positions within the area.
References herein to gantry systems are to be understood as including multi-axis positioning systems whereby a device is required to be accurately positioned in a two dimensional plane or a three dimensional space. In a typical two dimensional arrangement, the device is supported by a carriage which is movable back and forth in a first direction along a gantry beam. The gantry beam is movable back and forth in a second direction which is typically perpendicular to the first direction. The gantry beam is typically supported at both ends by a pair of carriages. If movement in three dimensions is required, the device is movably supported on the carriage so that the device is moveable in a third direction which is typically perpendicular to both the first and second directions. The three directions are typically orthogonal XYZ axes.
Travel distance, speed, acceleration, accuracy of placement and reliability are relevant factors for consideration in the design of gantry systems. Accuracy of placement and repeatability are especially critical for demanding applications where a tool or device must be positioned accurately with only a small margin for error. Conventionally, gantry systems utilized ball screw-based mechanisms and AC servomotors for driving the gantry. However, ball screws have inherent drawbacks such as relatively slow speed and lower precision.
More recently, linear motors have been introduced for driving the gantry systems and these have significantly improved performance, speed and reliability as compared to conventional ball screw systems. An example is U.S. Pat. No. 6,798,088 entitled, “Structure for Symmetrically Disposed Linear Motor Operated Tool Machine”. The gantry structure comprises two sustaining walls erected in parallel and a movable gantry that can reciprocate along slide rails laid on the sustaining walls. The movable gantry is driven by symmetrically-disposed high output linear motors. A disadvantage of such a design is that it does not cater to thermal expansion during operation and possible asynchronous operation of the motors driving the beam. As the gantry structure is quite rigid, the slide rails will encounter stress when the motors drive the gantry by an unequal distance or unequal force. Positioning accuracy will be affected and the slide rails will also face excessive loads and greater wear.
To address this problem, some flexibility may be introduced to the interface between the linear guides and the movable gantry, such as in U.S. Pat. No. 6,852,989 entitled, “Positioning System for Use in Lithographic Apparatus”. A positioning system that is used to position a movable object table in three degrees of freedom is described. The movable gantry is coupled rigidly to sliders mounted on parallel side beams in at least two axes to form a rigid body in the horizontal X-Y plane. A thrust bearing is pivotally mounted to at least one slider relative to a side beam for transmitting forces in the X-Y plane and perpendicular to the respective side beam between the movable gantry and the side beam.
If the gantry is displaced to effect yaw positioning of the beam, the linear motors of the sliders will be correspondingly rotated relative to their tracks. This compensates for thermal expansion and asynchronous operation, but on the other hand, it introduces various design complexities. Furthermore, the design introduces an extra degree of freedom of movement to the gantry to alleviate high stresses on the linear guides during yaw by providing extra rotary degrees of freedom at both ends. Unfortunately, an excess of rotary degrees of freedom leads to the gantry system having limited stiffness to counter roll and pitch movements, which ought to be high in order to achieve the requisite accuracy and dynamic performance.
It would be desirable to keep the overall design of the gantry system as simple as possible by assembling the system with fewer parts, and yet be able to achieve high positioning accuracy for the gantry.